This invention relates to friction clutch arrangements for motor vehicles. British Patent No. 2,256,907 discloses a friction clutch arrangement having a clutch disk which can be locked between two components and which is rotationally fixed to a transmission shaft, a clutch spring for locking the clutch disk between the two components, a clutch actuating device with a slave device situated near the clutch, a master device remote from the clutch, and a force transmission system between said two devices. In this arrangement, the slave device located near the clutch includes a fixed housing concentrically surrounding the transmission shaft and having two radially spaced axial extensions which are arranged concentrically to form an annular space and a piston that is movable within the annular space and is frictionally connected to the clutch disk. The slave device includes a piston motion sensor with a variable resistance, a variable capacitance, or a variable induction. If a piston sensor with a variable resistance, a variable capacitance or a variable inductance is provided, two cables must be connected to the slave device. Routing of the cables presents a problem, particularly with parts that move with respect to each other. Another problem of the two piston sensors of that type is that many moving metal components are present in the slave device which can influence the variable quantity that produces the measurement signal, such as capacitance, resistance or induction. Moreover, the function of the piston motion sensor of this arrangement can be impaired by contaminants such as constituents of the clutch lining or oil escaping from the engine compartment. The piston sensor described in this reference which has a variable induction, is made of ferromagnetic material or has ferromagnetic components. In addition to the above-mentioned disadvantages of piston sensors with variable capacitance, a further disadvantage of all such sensors is that they are placed close to the friction clutch and can be moved or oscillated by operation of the clutch. These imported motions also influence the measurement signal.
U.S. Pat. No. 4,705,151 also discloses a friction clutch arrangement in which a slave cylinder of a slave device has a piston sensor with a variable inductance, capacitance, resistance or induction. The piston sensor described therein is only provided to indicate an extreme position of the piston, however. Detection of the piston position over the entire displacement path is not possible with such a sensor.